Airfoils of turbine blades and vanes of gas turbine engines often require a complex cooling scheme in which cooling air flows through the airfoil and is then discharged through carefully configured cooling holes in the side wall of the airfoil and/or its associated structures. The performance of a turbine airfoil is directly related to the ability to provide uniform cooling of its external surfaces. Consequently, control of cooling hole size and shape is critical in many turbine airfoil designs, because the size and shape of the opening determines the amount of flow exiting a given opening, its distribution across the surface of the component, and the overall flow distribution within the cooling circuit that contains the opening. Other factors, such as back flow margin (the pressure differential between the cooling air exiting the cooling holes and combustion gas impinging on the airfoil) are also affected by variations in opening size.
Conventional hole drilling techniques include laser machining and electrical-discharge machining (EDM). These techniques yield airfoil castings with dimensionally correct openings in order to repeatably control opening size.
Water jet drilling is another versatile drilling method for precision drilling operations. However, conventional water jet drilling is primarily performed on structures that do not have a shallow drop through region. This is due to the physical limitations of being able to stop the drilling jet before it hits an opposing surface. While sacrifices could be made to allow for the opposing wall to be part-drilled, this would necessarily result in a decrease in part life and field performance. Such a sacrifice is illogical where other drilling techniques that do not decrease part life are available. Thus, water jet drilling is generally considered unsuitable for drilling nozzles and buckets. Nevertheless it would be advantageous to provide a method for drilling airfoil cavities with a water jet in a manner that avoids damage to an adjacent wall, once the hole has been drilled through and before the application of the jet is terminated.
The invention is embodied in a method for water jet drilling structures, such as nozzles and buckets used in gas turbines, wherein the opposite wall and/or adjacent structures are shielded from the water jet by providing a backer insert as a jet-stop to prevent unwanted erosion or drilling of the airfoil structure.
In one embodiment of the invention, in order to increase the durability of the backer, the backer is at least one of formed from or coated with a material that wears at a slow rate. More specifically, any water jet blocking material that is more resistant to water jet drilling forces than the material of the structure being drilled may be used to advantage in a method and/or insert embodying the invention. An exemplary material that may be adopted for the backer of the invention is carbide. Carbide by its physical nature is slow to wear, thus offering the durability required in part to part processing in any manufacturing environment.
According to a first aspect of the invention, a method is provided for drilling holes in a wall of a component having a hollow interior cavity, the method comprising disposing a backer insert comprising a water jet blocking material in the hollow interior cavity adjacent to a back surface of the wall to be drilled; water jet drilling at least one hole in the wall through to the hollow interior cavity; and removing the backer insert.
In one embodiment, the backer insert is formed from or coated with a blocking material, such as carbide, that is more resistant to water jet drilling forces that the material of the component wall.
According to another aspect of the invention, a backer insert is provided for being disposed in a cavity of a gas turbine component to intercept and disperse a water jet for drilling of a hole through a wall of the cavity, the backer insert comprising an insert component having a configuration generally corresponding but smaller than to a configuration of a back surface of the wall of the cavity. The insert component is formed from or coated with a water jet blocking material. In one embodiment, the blocking material is more resistant to water jet drilling forces that the material of the wall being drilled.